Dispersivity measurement of magnetic powders using the magnetic noise method (abstract)

Abstract

In spite of the different nature of high coercivity in hexaferrites (Ba, Sr, Pb) and intermetallic compounds (Sm CO5, Fe-Nd-B) the grain size distribution function in the powders is of a great importance for both types of sintered magnets. The conventional techniques in many cases could not provide a reliable estimate of the powders’ dispersion. The fine particles (particularly the single-domain ones) are magnetized almost to saturation and therefore are stretched together into the flocculas. These flocculas are taken by the various methods as a large particle, i.e., the methods fail. On the contrary, the jumplike domain wall motion parameters, obtained in BN spectrum study, let us analyze the distribution function of fine Ba-Sr hexaferrites powder particles by size. Later it was found that such dependence of the BN spectrum G(f) on the mean grain size dm is true for a wide variety of the powders whose dispersivity is far from a single-domain one. The stable dependencies of BN parameters on a powder dispersity was observed for a wider range in Ba-Sr ferrite 0.5<dm<4 mkm (since the single-domain size is about 1.3 mkm) and in Fe-Nd-B powders with 1.5<dm<20 mkm, and in pulverized iron powders with 30<dm<400 mkm and in fine electrolytic dendrite iron particles with a mean long axis size of 0.1<lm<0.2 mkm. The applied field and G(f) values are quite different in the above cases, but the main features remain the same. So the method may be widely used in various branches of magnetic powder and related technology for easy, fast, and reliable measurement and analysis of the distribution function of the powders’ particle numbers and specific volumes by size. We have developed a complete range of devices for the SPDA (system for powders dispersion analysis) equipped with a PC and proper software for analyzing and displaying the information obtained.